Carbon nanotube has a larger specific surface area compared to activated carbon, and therefore has been considered to be used as a carbon fiber membrane in a polarizable electrode of an electrical double-layer capacitor or the like. On the other hand, carbon nanotube is expensive and the increase of manufacturing cost of the polarizable electrode of the electrical double-layer capacitor or the like is unavoidable if only carbon nanotubes are used.
In this regard, conventionally, there is known a carbon fiber membrane in which carbon nanotubes and carbon material other than carbon nanotubes which is more inexpensive are combined by a binder (for example, refer to Patent Literature 1 and Patent Literature 2).